The invention relates to a metal gasket, such as a cylinder head gasket, to be sandwiched between two members, for example a cylinder head and a cylinder block, of an internal combustion engine to seal therearound.
In case joint surfaces of a cylinder head and a cylinder block (cylinder body) of a vehicle are sealed, combustion gas, cooling water, lubricating oil and the like are sealed by sandwiching a cylinder head gasket therebetween.
To meet demands of reducing weight of an engine and a production cost, a cylinder head gasket has been changed to a simple structural cylinder head gasket formed of one or two metal base plates from a metal laminate type gasket including a plurality of metal base plates. Therefore, the constituent plates become one or two, usable materials are limited from the aspect of making an engine light, and the kind and number of sealing devices are also limited, which forces to use relatively simplified sealing devices.
Therefore, mainly for a cylinder head gasket, for example as shown in FIG. 13, there has been proposed a gasket 2, wherein a main bead 6A and sub-beads 6B linearly connected to the hem portions of the main bead 6A are provided to a metal plate 2 so that surface pressures are generated concentrically at top portions of the main bead 6A and the sub-beads 6B to positively seal with large surface pressures (For example, Japanese Patent Publication (KOKAI) No. 11-230355, refer to page 2, FIG. 2).
Also, as a gasket formed of two plates, as shown in FIG. 14, there has been proposed a metal laminate gasket for sealing a port of an intake-exhaust system of an engine, wherein a sectional shape of a circular bead 4 surrounding the port 2 is formed in a wave shape having two continuous mountain-shape portions 4a, 4b to project mutually in opposite directions from flat portions of a metal plate 1, and the two metal plates are laminated in such a manner that the top portions of the mountain-shape portions 4a of the circular beads 4 abut against each other. Therefore, the metal plate 1 is provided with the circular bead 4 having two heights of the mountain-shape portions, and the circular beads 4 of the respective layers are sufficiently subjected to an elastic deformation to thereby effectively prevent leaking of a medium to be sealed (For example, Japanese Patent Publication (KOKAI) No. 2002-54502, refer to page 3, FIG. 2).
However, in the gaskets having such wave-form circular beads, while they are effective for concentrating the surface pressure or enlarging the crushed margins, the rigidity is basically determined by the sectional shape of the beads formed on the plate.
On the other hand, in a cylinder head gasket and the like, even in case the same gasket is used to seal between the same engine members, the sealing performance required by the kind of a hole to be sealed becomes different. For example, in a hole for a cylinder bore, it is required to seal a combustion gas having a high temperature and a high pressure, while in a liquid hole for circulating cooling water and engine oil, it is required to seal a liquid having a comparatively low temperature and pressure.
Also, from a structural reason of an engine, there are generated different surface pressures in a hole for a cylinder bore surrounded by bolt holes for tightening bolts and a liquid hole located on an outer side so that the pressing forces by the tightening bolts are only applied to one side thereof.
In the metal gasket as described above, in case a bead is simply provided, the rigidity of the bead is determined by the shape of the bead, material characteristics of metal of a base plate and a thickness of the base plate. Therefore, the design freedom is limited, and it is difficult to provide the optimum sealing performance with respect to the respective holes to be sealed.
Especially, since a high surface pressure is required around the hole for the cylinder bore, in order not to damage the engine members made of a relatively soft aluminum alloy, it is difficult to use a metal base plate having a high rigidity. On the other hand, there is also a problem that a sufficient rigidity can not be obtained with the conventional shape and arrangement of the bead.
Also, as shown in FIG. 14, in the gasket where the top portions 4a of the mountain-shapes of the wave-forms in a two plate structure abut against each other, in case the abutting beads are mis-aligned, since a predetermined compression rigidity of the beads 4 may not be obtained, it is required to carry out the precise alignment of the abutting beads 4.
However, in a cylinder head gasket and the like having a fine bead of a width less than 3 mm, the positioning is actually difficult, so that an advanced technique is required.
In view of the above, the present invention has been made, and an object of the invention is to provide a metal gasket of a two plate structure excellent in sealing ability, such as a cylinder head gasket, wherein sub-beads are respectively provided around main beads disposed to respective metal base plates for sealing a hole to be sealed and the sub-beads abut against each other so that an adequate bead rigidity can be obtained by constraining the deformation in a radial direction of a main bead of one metal base plate by the sub-bead of the other metal base plate. Thus, the optimum surface pressure distribution can be generated around the hole for the cylinder bore or the like and maintained.
Further objects and advantages of the invention will be apparent from the following description of the invention.